THE EFFECTS OF ENVIRONMENT AND 

 NUTRITION UPON FERTILITY 



BY FRANCIS H. A. MARSHALL, M.A., D.Sc 

 Lecturer on the Physiology of Reproduction in the University of Edinburgh 



Experienced breeders of animals have seldom failed to recognise 

 that the generative system is particularly susceptible to changed 

 conditions of existence. Buffon long ago commented on the 

 fact that the domestic animals are as a rule far more prolific than 

 their wild representatives, and Darwin, who made the same 

 observation, was disposed to ascribe the increased fertility of 

 the former to a long habituation to a copious food supply 

 without the labour of seeking for it. On the other hand, it is 

 equally well known that wild animals when brought into 

 captivity often become sterile, while there are numerous 

 instances also of domestic animals failing to breed as a result 

 of removal to a new environment. Such sterility, when it occurs, 

 is not necessarily (or even generally) associated with disease 

 or want of vigour, for, as Darwin observed, wild animals in 

 confinement are not infrequently notoriously healthy and 

 long-lived, and nevertheless fail consistently to produce young. 

 There is no evidence that the reproductive organs are them- 

 selves diseased, yet they seem incapable of performing their 

 natural functions. Moreover, in those cases in which wild 

 animals in captivity have proved fertile, the capacity to breed 

 is not necessarily inherited by their offspring, although, as a 

 general rule, the fertility tends to increase in subsequent 

 generations. Thus, it was a considerable time before the 

 canary bird was fully fertile, but it eventually became so, 

 whereas certain of the nearly related finches, while producing 

 hybrids with the canary, only occasionally reproduce their own 

 kind when kept in captivity. Other wild birds again, such as 

 most members of the great families of ducks, pigeons, and fowls 

 breed as readily in confinement as in their natural state. 

 Similar differences occur in the various groups of mammals. 

 For instance, bears breed less freely in the Zoological Gardens 



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